<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }

        ul,
        li {
            list-style: none;
        }

        #box1 {
            width: 200px;
            height: 400px;
            background: #ccc;
            padding: 20px;
            box-sizing: border-box;
            margin: 100px auto;
        }

        #li_list {
            width: 160px;
            height: 250px;
            display: flex;
            justify-content: space-between;
            flex-wrap: wrap;
        }

        li {
            width: 50px;
            height: 50px;
            color: #ffffff;
            background: #111;
            line-height: 50px;
            text-align: center;
        }

        li:hover {
            background: #58bc58;
        }

        #box {
            width: 160px;
            height: 100px;
            background: #ffff;
        }
    </style>
</head>

<body>
    <div id="box1">
        <ul id="li_list">
            <li>1</li>
            <li>2</li>
            <li>3</li>
            <li>4</li>
            <li>5</li>
            <li>6</li>
            <li>7</li>
            <li>8</li>
            <li>9</li>
            <li>10</li>
            <li>11</li>
            <li>12</li>
        </ul>
        <div id="box"></div>
    </div>
</body>
<script>
    (function () {
        var li_list = document.querySelectorAll('#li_list li');
        var box = document.getElementById('box');
        var ev = ['1月', '2月', '3月', '4月', '5月', '6月', '7月', '8月', '9月', '10月', '11月', '12月']
        box.innerHTML = ev[0];
        for (var i = 0; i < li_list.length; i++) {
            li_list[i].index = i;
            li_list[i].onmouseover = function () {
                box.innerHTML = ev[this.index];
            }
        }
    })();
</script>

</html>